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A couple weeks ago, I got the "terrible triad" of lights (ABS, 4x4, and Brake). Recently, I got the clicking noise after turning the car off and was able to determine that it was coming from a transfer case. Using several posts from here, especially this one:

I bought a new transfer case actuator and installed it. Problem solved, no lights, no clicking, all better. It is not a cheap part at 720 bucks from getbmwparts.com, and 900 bucks elsewhere, so the engineer in me wanted to figure out why they were failing.

Which brings me to the purpose of this post. I disected the old actuator to find out what the heck is going on inside it. I appologize for the huge pictures, but some sort of resolution was needed to describe what I was trying to say.

What the actuator looks like when off the car:

And broken down into the pieces: Bottom right is the motor, top right is the brush assembly, top left is a guard of some sort that pops off (albeit bent and unusable once you have it off) and the bottom left is the gear assembly (we'll focus on this one)

After I popped off the oil guard piece, here is what the inside looked like, a ton of shavings and grease.

and closer:

So you can see the motor turns a worm gear which is mated to a black gear (which is made of plastic !!!) Eventually, this gear wears down like crazy and doesnt have any teeth left to grip the worm gear. This is what causes the slipping, and the error lights, and especially the clicking sound.

In this photo, you can see the change from good gear area to the bad area. The good area is on the left, and as you follow the black gear around to the right, you can see where it has been worn down from sitting all the time.

So, I went farther. I took off the black gear to see what it looked like:
Good side (what it should look like everywhere):

Bad side (Cause of the problem)

POTENTIAL SOLUTION:

So now we know what the cause is, how can we fix it? It has been (from my research) that everyone just buys a new actuator. Yes, it fixes the problem, I can vouch for that, but I don't know how many other people looked into what CAUSED the problem.

I bet you could unbolt the motor from the gear housing assembly (it is held on by 4 torx bolts that come off quite easily compared to trying to remove the actuator from the car).
Mark a location of the current position on the white plastic gear you can see in the first photo. After the motor is unbolted and removed (ONLY THE MOTOR, DONT DESTROY YOUR ACTUATOR AND TAKE THE WHOLE THING APART!!!!), turn the white plastic gear 180 degrees from its current location. This takes the bad part of the gear and puts it where it will not get used, which means that a good part is now in contact with the worm gear.

Re-insert the motor and bolt back together. This should give you, say another 65k miles (when mine failed) or so before you likely have to replace the actuator. Re-install on your transfer case, and voila! you should be good to go with only an hour or so of your time wasted instead of time and 900 bucks.

I haven't tried this, so try it at your own risk, but with pulling everything apart, it seems like a pretty safe bet, but obviously I can't be held responsible if something does go wrong.

Hope perhaps it can save someone almost a thousand dollars. I believe my logic is sound, but I look forward to your comments!

OK. Engineer here. Not quite following your description of the gear reversal and how that 'fixes' the problem, so maybe I'm missing something critical, but:

I don't think reversing (or simply rotating, if that's what you're suggesting) the exposed white gear 180 degrees will correct the problem. Yes, that could re-position a 'good' portion of the black gear onto the worm gear, but regardless of how the gears are positioned, as all of them turn, the black gear will rotate until its worn portion reaches the worm gear, then it stops turning again. This will happen within one rotation of the black gear.

Based on the wear that you highlighted on the black gear, I think it would have to be replaced -- if one can find the part...

OK. Engineer here. Not quite following your description of the gear reversal and how that 'fixes' the problem, so maybe I'm missing something critical, but:

I don't think reversing (or simply rotating, if that's what you're suggesting) the exposed white gear 180 degrees will correct the problem. Yes, that could re-position a 'good' portion of the black gear onto the worm gear, but regardless of how the gears are positioned, as all of them turn, the black gear will rotate until its worn portion reaches the worm gear, then it stops turning again. This will happen within one rotation of the black gear.

Based on the wear that you highlighted on the black gear, I think it would have to be replaced -- if one can find the part...

It looked to me that the black gear doesn't turn the whole way around, only really over that worn area. I believe BMW used a full gear as to ease the installation (not having to worry about the position you put it in)

If it continuously rotated around, woundn't the gear wear more evenly and not in a single spot about the length of the worm gear?

Again, I guess it might not be a fix per say, but it could seriously delay the need to spend 900 bucks.

It looked to me that the black gear doesn't turn the whole way around, only really over that worn area. I believe BMW used a full gear as to ease the installation (not having to worry about the position you put it in)...

Maybe I need to fiddle with it myself, like you have, to really see what's going on there. But what you're describing wouldn't be consistent with how a worm gear and a cog gear typically interact.

Quote:

Originally Posted by pburnett

... If it continuously rotated around, woundn't the gear wear more evenly and not in a single spot about the length of the worm gear...?

It may have worn evenly until it failed; you'd have to compare the teeth on the gear you extracted from your actuator to a brand new gear.

But I think what's happening is that these plastic gears are not durable enough; they either have a weak spot or just get 'chewed up' by the metal worm gear over time. I'm actually rather shocked that BMW even used plastic cogs inside a transfer case, but...

All is not lost, though; you did a good job in isolating the weak component in the actuator! Hopefully someone can zero in on a supplier for those black gears (they've gotta come from somewhere...), and we can all re-build our failed xDrive actuators!

But I think what's happening is that these plastic gears are not durable enough; they either have a weak spot or just get 'chewed up' by the metal worm gear over time. I'm actually rather shocked that BMW even used plastic cogs inside a transfer case, but...

I was surprised that even garage door opener motors have those nylon worm drive gears.

Recently took apart one - same sort of thing. The plastic just gets badly chewed up in one section.

The repair guy said it happens randomly, through bad luck.
They otherwise work fine for well over 10 years.

As you say, a weak point in the plastic, or maybe things get slightly out of alignment and the damage is started.

May be the same for the transfer case actuator - some will have that problem, but not inevitable.

Maybe I need to fiddle with it myself, like you have, to really see what's going on there. But what you're describing wouldn't be consistent with how a worm gear and a cog gear typically interact.

I see what you are saying. I figured that the gear had, say, 90 degrees of freedom in rotation (on/off on either side of the 90) for actuated/non-actuated. So the real question when the actuator is active, does it spin the black gear constantly or does it stop at some point? I have no idea. Really, it is because I don't know what or how the inside of the X3 transfer case works or looks like, but I am beginning to think you may be correct. Unfortunatly, I don't see a way to get to the black gear without doing some serious damage to the cover that I pulled off, so even if you had found a replacement, I don't think you can get to it to just replace it, but maybe with some luck you can.

Just for reference, I attached kind of what I was talking about in the picture. I'm just wondering if maybe the motor goes forward/reverse to turn on and off something with the transfer case. something like this would cause only this area to be used. Not the best solution for it, but likely cheap and easy to manufacture.

I see what you are saying. I figured that the gear had, say, 90 degrees of freedom in rotation (on/off on either side of the 90) for actuated/non-actuated. So the real question when the actuator is active, does it spin the black gear constantly or does it stop at some point? I have no idea. Really, it is because I don't know what or how the inside of the X3 transfer case works or looks like, but I am beginning to think you may be correct. Unfortunatly, I don't see a way to get to the black gear without doing some serious damage to the cover that I pulled off, so even if you had found a replacement, I don't think you can get to it to just replace it, but maybe with some luck you can.

Just for reference, I attached kind of what I was talking about in the picture. I'm just wondering if maybe the motor goes forward/reverse to turn on and off something with the transfer case. something like this would cause only this area to be used. Not the best solution for it, but likely cheap and easy to manufacture.

Your theory makes sense to me, mostly since Xdrive is RWD most of the time. And it transfers power to the front as needed, it seems like the electric motor turns more the gear, as more power is transferred to the front, wearing the "most used" area considerable more.

What I think you are looking at in the top picture, top right hand side is a brushless DC motor, which would be controlled by output from a computer that would tell it to turn a certain number of revolutions in a specific direction. It is likely that the drive gear never does rotate 180°, so rotating the drive gear may work.

The concern would be why the gear wore out. It is obviously being overloaded, which could either be a processing problem in molding the gear or it could be that whatever the actuator is trying to turn is not turning a freely as it is supposed to.

At any rate, it would definitely be worth a try. The only downside I see is expending the time to remove the actuator.

The xDrive PDF referenced does attempt to explain how the servo motor on the transfer case works to actuate the clutch packs in the transfer case.

Yes, I meant to say rear-bias. Basically what this actuator does is to tell the TC how much torque to pass onto the front wheels. So the theory that doesn't turn 360* sounds right.

Or maybe it *could* turn more but only on cases when full traction is needed to the front wheels which will depend on the individual driving conditions. Maybe this "spot" is the one that gets more use, therefore wearing out faster.

It would be nice if they'd sold an "Actuator Rebuilding kit" instead of selling the whole unit as a replacement.

I see what you are saying. I figured that the gear had, say, 90 degrees of freedom in rotation (on/off on either side of the 90) for actuated/non-actuated. So the real question when the actuator is active, does it spin the black gear constantly or does it stop at some point? I have no idea. Really, it is because I don't know what or how the inside of the X3 transfer case works or looks like, but I am beginning to think you may be correct. Unfortunatly, I don't see a way to get to the black gear without doing some serious damage to the cover that I pulled off, so even if you had found a replacement, I don't think you can get to it to just replace it, but maybe with some luck you can.

Just for reference, I attached kind of what I was talking about in the picture. I'm just wondering if maybe the motor goes forward/reverse to turn on and off something with the transfer case. something like this would cause only this area to be used. Not the best solution for it, but likely cheap and easy to manufacture.

OK, I follow what you're saying now too.

Well, if the black gear only turns a quarter-turn or so at most, your idea could potentially work; how far the black gear rotates is going to depend on the reduction between it and the worm gear, and how much the black gear needs to turn in order to adjust the torque transfer between the axles...

Well, if the black gear only turns a quarter-turn or so at most, your idea could potentially work; how far the black gear rotates is going to depend on the reduction between it and the worm gear, and how much the black gear needs to turn in order to adjust the torque transfer between the axles...

Yup.... And thats a big IF, and I don't know the answer. But if someone has this problem and gives it a try, we may find out more.

I'm really curious as to if it would work. It might, it might not. Do you have the clicking noise when you turn off the car from underneath?

To do the "fix", all you have to do is unbolt the actuator motor (I think you said you did this already), turn the white gear 180 degrees, and put the motor back in. The worm gear will seat itself properly (I have tried this with my messed up one, so you should have nothing to worry about). Reinstall and then let us know!

If you can drive a few times and try to push the X-drive system so that it has to work, and report back many of us would be very eager to hear how it works. Good luck!

I see good logic in both of those. It will be interesting to see if more people can have success with this. Either way, I would mark the bad area somehow so you know which way you turned it. I could see 1/3 turn working, but I wouldn't go much less than that.

been driving the whole day without any problem and without the lights coming back on
and no clicking sound when turning the engine off.

Thanks for the help this was a successful experiment that save me lot of $$$$$$$$

Well, sorry to rain on the parade, but don't celebrate just yet...

Are you certain that you fully exercised the xDrive so that the full range of torque transfer was explored? If not, you might discover that the black gear will end up slipping at some stage -- like during inclement weather...

I recommend cautious optimism. Hopefully "pburnett's" idea will stand the test of time, but let's see how your system handles this 'shade tree repair' as winter sets in... Honestly, I hope it bears itself out successfully!

Are you certain that you fully exercised the xDrive so that the full range of torque transfer was explored? If not, you might discover that the black gear will end up slipping at some stage -- like during inclement weather...

I recommend cautious optimism. Hopefully "pburnett's" idea will stand the test of time, but let's see how your system handles this 'shade tree repair' as winter sets in... Honestly, I hope it bears itself out successfully!

I agree 100%. I'm optimistic now, but the true test would be winter, and unfortunatly, as the poster who did the fix is in Puerto Rico, we may not see it. I recommend finding some loose dirt or gravel roads and make the xDrive work. Then, we will see if it truely is possible to delay buying a new actuator.